In railroad freight cars, the center plate on the underside of the car body sits inside the center bowl on the truck bolster and is guided into position during assembly via the use of a center pin, or kingpin, that fits through the holes formed in the center bowl and the center plate. Typically, a liner is placed between the center plate and the center bowl surfaces to help reduce wear and/or modify the turning resistance of the truck. Historically, this liner has taken the form of a metallic horizontal disc that may also be used in conjunction with a metallic ring or an upstanding cylindrical wall at the outer perimeter of the disc to protect both the horizontal and vertical surface of the center bowl and center plate. However, these metal liners typically need to be lubricated periodically, which is burdensome and expensive.
In recent decades, railroad freight car builders and owners have started utilizing non-metallic horizontal or cup shaped liners. Some examples of non-metallic lines are the ultra-high molecular weight (UHMW) polyethylene and glass-filled polyester thermoplastic urethane (TPU) center bowl liners currently in the market today. These non-metallic center bowl liners can provide some benefits over traditional metallic liners, such as lower cost, ease of installation, lower coefficient of friction, reduced wear, etc. However, they are not without their own issues.
One drawback with the use of a non-metallic liner is that when a non-metallic liner is added between the car and the truck, there is no electrical continuity or conductivity between the car and the truck. Electrical conductivity between the car and the truck is important to provide a ground for the car body. Thus, use of a non-metallic liner can create an environment where static electric charge can build up in the car structure, resulting in a dangerous and unsafe condition. To combat this, manufacturers typically add a metallic or conductive material to the liner, such as metallic buttons or carbon black to provide some means of conductivity between the car and the truck to prevent the buildup of static electricity. Unfortunately, carbon black is not as conductive as metal and can be considered only an anti-static or semi-conducting material. On the other hand, while the metallic buttons provide sufficient conductivity, they can lift the car body out of the bolster bowl if they don't crush properly or they can become loose over time and fall out of the center bowl liner.
In addition, the non-metallic center bowl liner material itself is subject to failure. If the liner has a coefficient of friction that is too low, it can melt due to frictional heat generated by hunting, which is a violent oscillation of the truck relative to the car body. Furthermore, some materials, such as UHMW polyethylene, are prone to compression set, or “cold flow”, when subjected to high compression loads, which can reduce the life of the component. As a result, some suppliers have chosen to add glass fibers to the non-metallic liner material to improve its compression set characteristics. However, under certain loading conditions, the glass can be abrasive and increase the rate of wear in the center bowl or center plate. Finally, UHMW polyethylene and glass-filled polyester TPU materials can become brittle and crack at low temperatures.
The present invention is directed toward overcoming one or more of the above-mentioned problems.